Japanese Unexamined Patent Application Publication No. 11-11267 discloses a foldable mobile radio communication device. In communication devices of this type, unless otherwise a key operation section is projected from a surface of a lower housing of the device, operability of the key operation section is deficient. As such, the foldable mobile radio communication terminal device is provided with a mechanism that causes respective keys of the key operation section to project from the surface section of the lower housing when the communication device is operated to an open state, thereby to enhance the operability of the respective keys of the key operation section.
However, in the case of a foldable mobile phone (mobile terminal device), when the mobile phone is in a closed state, an upper housing and a lower housing closes in opposition to each other to a state where the entirety of the key operation section, which is provided on the lower housing side, is covered by the upper housing. There arises a problem in that wasteful operation is necessary to rotationally operate the upper housing or lower housing by about 180° about a hinge portion on the rotation axis so that the mobile terminal device is put into an open state thereby to cause the key operation section to be exposed.